1. Field of the Invention
This invention relates to the field of air ionization, which is used to remove static charge from objects in critical process environments. Critical process environments include manufacturing or research facilities for semiconductors, disk drives, flat panel displays, optoelectronic devices, and nanotechnology processes. Positive and negative ions created by the air ionization are attracted to statically charged surfaces with opposite charge. Hence, surface charge is neutralized.
Gas (or air) molecules are ionized with sharp electrodes to which high voltage is applied. Ions are carried away from the sharp electrodes and toward the target (the object to be discharged) by electrostatic forces.
Properly designed addition of pressurized gas (or air) results in a performance improvement, relative to a same self-balancing shielded bipolar ionizer without pressurized gas. Performance parameters include discharge time, effective functioning distance, and balance.
2. Description of Related Art
The subject matter of this disclosure is an improvement to U.S. Pat. No. 6,002,573, issued Dec. 14, 1999 to inventor Leslie W. Partridge, entitled “Self-balancing Shielded Bipolar Ionizer”. Both this current invention and U.S. Pat. No. 6,000,573 are assigned to Ion Systems, Inc. and commonly owned by Ion Systems, Inc. in Berkeley, Calif.
U.S. Pat. No. 6,000,573 describes a self-balancing air ionization system based on at least two electrodes (and normally four electrodes) positioned within a recessed cavity. The recessed cavity is open only in the direction of ion production, which corresponds to the direction that the electrode tips are pointed. The electrodes are placed close together to achieve self-balancing. These self-balancing air ionizers are relatively small in size. Dimensions are 3.6″×1.4″×1.4″ in the most common embodiment. However, these dimensions are not presented as a size limitation. Balance is achieved by generation of equal numbers of positive ions and negative ions. Insulative surfaces of the recessed cavity repel ions to be carried to target area by the ion current.
This prior art works well to remove static charge. But the useful operating distance between the ionizer and the object to be discharged depends upon airflow within the working environment. When environmental airflow is slow or stagnant, ionizer operation more than 6 inches from the target is marginally effective. Ions of opposite polarity recombine before reaching the target. Static charge neutralization requires long exposure times.
Dependence on environmental airflow makes it difficult to predict performance. Airflow patterns inside process equipment are not always known. Turbulence and reverse flows may carry the ions away from the target, rather than to the target. Positioning the ionizer in the best location involves guesswork or experimentation. Hence, the performance of the prior art self-balancing shielded bipolar ionizer depends on factors beyond the manufacturer's control.
One way to maximize performance of the prior art is to position the ionizer close to the target. But this involves risk. If an electrode gets too close to the target, localized charges (“hot spots”) can be created on the target. This is counter-productive.
Prior art performance improvement can also be addressed at the design stage. For example, placing a higher voltage (or current) on the electrodes is possible. But higher voltage (other factors constant) results in higher particle generation, which is undesirable in clean processes.
Purity of environmental air is also a performance factor. If the environmental air contains impurities, these impurities can react with the electrode tips to form undesirable buildup. When this happens, cleaning is needed to restore the original discharge time, balance, and cleanliness. Depending on clean environmental air introduces another uncontrolled variable. The goal of this invention is to increase effectiveness (shorter discharge time, balance uniformity, longer time between maintenance) on this ionizer by providing purging air while preserving size, self-balancing and serviceability of a present design.